The present invention relates to an motional feedback loudspeaker, and more particularly to an MFB loudspeaker designed to suppress peaks on frequency characteristics due to resonance of a diaphragm.
MFB loudspeakers are designed to control motions of a vibratory system of the loudspeaker by feeding a voltage proportional to the motions of the vibratory system back to an input of an amplifier serving as a driver of the loudspeaker. The MFB loudspeakers have reduced distortions of the vibratory system and improved sound-pressure vs. frequency characteristics. With this type of loudspeaker, a sufficient MFB effect is available for vibrations at any point while the vibratory system is vibrating in unison at all times. At high frequencies, however, the vibratory system has parts vibrating differently and cannot be controlled with one voice-coil type driver. Therefore, the MFB effect is effectively only in the region in which the vibratory system vibrates in unison.
Various devices for detecting vibrations of the vibratory system are known. They include an acceleration pickup employing a piezoelectric element for detecting the acceleration of the diaphragm, a speed detection coil for the speed, and an electrostatic pickup for detecting the amplitude of vibrations. FIGS. 1 and 2 illustrate a loudspeaker incorporating an acceleration pickup. An acceleration pickup 3 is mounted substantially centrally on a flat diaphragm 2 supported on a baffle 1. The flat diaphragm 2 is supported peripherally by an edge 4 and has a central bobbin 5 with a voice coil wound therearound. The voice coil is positioned in an air gap in a driver unit 6.
The acceleration of vibrations of the flat diaphragm 2 is detected by the acceleration pickup 3, which produces a detected voltage applied through a negative feedback circuit to an input terminal of a loudspeaker driver amplifier.
To position the lead wires from the acceleration pickup or from the standpoint of a detecting position, the pickup is disposed near the voice coil or inside of the bobbin 5. When the loudspeaker is driven, the resonance mode of the flat diaphragm 2 suffers from a high peak at a particular frequency (in the vicinity of 650 Hz) as shown by the solid line in FIG. 3. The peak tends to cause the problem of oscillation when supplying a motional feedback signal to the loudspeaker. For supplying a stable feedback signal, a low-pass filter is employed as a feedback circuit to limit a frequency band to be fed back, or a band-rejection filter is used to reject signals in the corresponding frequency band.